A 3D printed Ronchi tester
The situation was that there were three telescopes. Each with the same 20″ cellular mirrors but one with a slightly different cell than the other two. Two were easy to collimate while the third one could never be fully collimated. It appeared that there was a bit of astigmatism that was preventing easy collimation. So, the problem was how to quickly test a mirror for astigmatism. A Ronchi test was suggested but I didn’t have a grating or even a tester. What to do? Print one? The first step was to find a grating. Since I couldn’t wait for a proper grating to arrive, I decided to try to print a 100 line/inch grating on transparency film with a laser printer. The second step was to make something to hold the grating and a light source. First I cobbled together something out of scrap plywood and, although it worked, it looked like something that the dog dragged in. It was…